Local-distant radioreceiver



Aug. 30, 1932. R. A. BRADEN LOCAL DI STANT RADIORECEIVER Filed April 50. 1950 (If/l/Ilf/M Mill/S INVENTOR RENE A BRADEN ATT Patented Aug. 30, 1932:

UNITED STATES PATENT OFFICE;

RENE A. BRADEN, or MEItCI-IANTVILLE, NEW JERSEY, AssIGNoR T0 RADIO conrorm- 1 TION OF'AMERIGA, CORPORATION OF DELAWARE LOCAL-DISTANCE RADIOBECEIVER Application filed April 30, 1930. Serial no. 448,509.

My present invention relates to radio receivers, and more particularly to a receiving circuit readily adapted to local and distant reception.

In designing an ideal radio receiver there should be taken into consideration, among other factors, the fact that the receiver should be capable of simple and speedy adjustment from local to distant reception. For local reception the receiver should give the best possible fidelity of reproduction. The sensitivity should be low; since high sensitivity is not necessary when strong signals are received fromnearby stations,and additional unused sensitivity makes the receiver more expensive to build.

For distant reception the sensitivity of the receiver should be very high, so that satisfactory operation can be had with very weak signals. The highest fidelity is not required because (1) Programs coming from a very distant station are generally somewhat distorted in their passage through space,'and therefore, a slight additional distortion in the receiver would not be noticed;

(2) Where very weak signals are being received, the noiselevel .is' so high, rela-. tively, that the station cannot-be listened to with pleasure; and moreover, persons who listen to such signals are generally not interested in the highest fidelity of reproduction. V V

A slight reduction in fidelity is desirable if it will permit a great reduction in the cost of manufacture for the reasons given above. The mechanism utilized for the change-over from local to distant reception should be of sin'iple construction, andreadily adapted to the requirements of the average receiving circuit. i

Accordin ly, the main object of the invention is to providea very simple and inexpensive receiver which will give either,

1) Very good fidelity, and enough sensitivity for local reception; or,

(2) Very high sensitivity, and fairly good fidelity; and, to provide an audio amplifier circuit which permits these two different characteristics to be attained in the same receiver.

Another important object of the invention is to provide a radio receiver including means for readily adapting the receiver to 6" local or distant reception, the said means comprising a resistance coupling between the output of a detector stage and the input of the first stage of audio amplification, said coupling being adjustable to automatically change the amount of audio frequency amplification, and thus change thesensitivity of the receiver to a high or low value.

Another object of the invention is to provide a receiver of signal modulated radio frequency energy, the receiver comprisin a detector provided with a biased contro electrode, a signal amplifier, and means coupling the detector and amplifier, saidcoupling means being adjustable to selectively 79 change the audio amplifier so that the amplification is high when receiving from Weak or distant stations, and low when receiving strong signals from local stations.

Still other objects ofthe invention are to improve'generally the simplicity and efficiency of radio receivers, and to provide a receiver readily capable of distant, as well aslocal, reception,' the receiver being reliable inoperation, and economical to manufacture. 80

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims, the

inventionit'self, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically one circuit arrangement whereby my invention maybe carried into effect.

Referring to the accompanying drawing there is shown, in diagrammatic manner, a radio receiver which consists of the usual grounded antenna circuit A, G, the antenna being coupled, as at M, to any type of well known tuned radio frequency amplifier, the latter being shown conventionally on the drawing for the reason that it is Well known to those skilled in the art that one or more stages of radio frequency amplification can be provided in a radio receiver prior to the detection of the signal modulated radio frequency energy. The amplified output of the radio frequency amplifier is impressed upon the input circult of a detector stage 1, in any Well known manner (tuned input, for example).

The detector stage consists of a'space discharge device, conventionally shown in the drawing as a triode 1, it being understood that any other type of multi-electrode device could be used in place thereof. The control electrodeof the triode 1 is biased negatively at a predetermined value by a source of biasing potential C. The output circuit of the triode 1 is coupled to the input circuit of the first stage of the audio amplifier by means of a resistance 6, a condenser 7 being employed to by-pass the source of anode current B.

. Anode potential is supplied to the anode of the triode 1 from a source. B, (not shown), the current from the anode source being fed through a lead 8 to one side of the resistance .6 so that the anode current flows through the resistance 6, to the anode of the. triode 1. An inductance 10 is connected in series between the anode of the triode 1 and the resistance 6, each terminal of the inductance 10 being grounded through a small capacity 11, the elements 11, 10, 11 forming a low pass filter whose purpose is to prevent radio frequency currents (which flow in the anode circuit of the tube 1) from reaching the grid of the tube 2. A further object of 11 (but not of 11) is to provide a low impedance path from the anode of tube 1 to ground. This arrangement is well known, and is used in most receivers. It has no effect except to allow'the detector to workat maximum efliciency, and to reduce the possibility of oscillation in the radio frequency amplifier. Y

The audio amplier is shown here as. consisting of two stages, the first stage comprising a space discharge device 2 of the triode type, the anode of which triode receives current from a source B (not shown), through the primary 11 of the audio transformer T. The control electrode of the triode 2 is nega tively biased by a source C", the negative terminal of the source being connected to the control electrode through a resistance 1, the positive terminal being connected to the cathode lead. The lower end of the primary 11" is connected to the cathode of the triode 2 through a capacity, the secondary 12 of the transformer T being connected to the cathode: of the triode 3, of the second audio stage,

through a capacity 12. The purpose of the small capacities is to provide low impedance paths to the cathodes for alternating. currents flowing in these windings, so the cur-- rents will not flow through the sources of anode and grid bias voltage. (This, it is well known, is likely to produce oscillation or distortion.)

The anode of triode 3 receives current from a source B (not shown), through a choke coil 14, the control electrode of the triode 3 being negatively biased from a source G (not shown). The capacity 15 is connected in series between the anode of the triode 3 and the primary 16 of the transformer T, the latter being connected to any well known type of utilization means such as a loud speaker, head phones, or the like. It will thus be seen that after the signal energy has been detected, two audio frequency amplifier stages are used to amplify the detected signals, the first amplifier stage being coupled by a resistance to the detector, and the second audio stage being coupled by a transformer to the first stage.

By means of a switch arrangement the amount of audio frequency amplification can be changed from a high value to a smaller value. This switch mechanism consists of a movable contact member 20 connected at one end to the control electrode of the triode 2, through a capacity 2, the other end being adapted for connection to a contact 4 connected to the anode and resistance 6, or to a contact 5 connected to a suitably determined point on the resistance 6. When the high value of audio amplification is employed, only a small input energy is required to the de- T tector triode 1 which has its control electrode biased. When the low value of audio amplification is utilized, a high detector input is required to make up for the reduction of audio amplification, the detector operating at this point as a linear detector, thus giving the'best quality of reproduction. This linear operating characteristic is obtained by adjusting the movable contact 20 so that it is connected to the contact 5. This aforementioned adjustment represents the low gain connection and is employed for local reception. It will, of course, be realized that for local reception it is desirable to have the best quality of reproduction, the reduction in audio amplification not being a disadvantage for the reason that the local signals are exceedingly strong.

. )Vhen the movable contact 20 is adjusted for connection to the contact 4, a high audio amplification connection results, and this connection is employed for distant reception, the quality of reproduction being impaired to a certain extent because with this connection the detector follows a square law operation.

As explained in detail heretofore, for distant reception it is more important to have a higher audio amplification available, than to have maximum quality of reproduction.

Without going into details the reason why the same detector tube can operate as either a linear or a square-law detector is explained by the fact that if the carrier voltage is adjusted to small value, the detector is working on the curved part of the characteristic curve, which examination shows to be a square-law curve; while. if the carrier is increased to a greater value, the operation takes place on the straight part of the curve. The latter is linear detection, while the former is square-law detection. Any linear detection constructed according to present knowledge, will operate as a square-law detector if the signal input is made very small. Thus, with low audio frequency amplification, highdetector input is required, and operation then occurs on the linear part of the detector curve, while with high audio frequency amplification, the detector input is low, and operation takes place on the square-law portion of the curve. p

The important difference between linear and square-law detection is that the latter introduces harmonics of the audio frequencies, while the former does not. The difference is not as great in practice as one might expect. Numerically, the percentage of second harmonic produced in a square-law detector is one fourth of the percentage modulation. A broadcasting station averages 25- 30% modulation, only the loud crashes going up to 80% or 100%. Therefore, the average second harmonic is only 67% except occasionally when the volume is very high. Moreover, so-called linear detectors are not perfect; generally they produce about half as much second harmonic distortion as the square-law detectors.

Therefore, while for local reception, it is worthwhile to use linear detection (if it doesnt cost too much), the extra distortion due to using square-law detection for distant reception is really not serious, and is justified when the saving in cost is considered. Linear detection could be used for distant reception, also, if the switch was connected so that it cut in an extra radio frequency amplifier stage. This is much more expensive however than adding an extra audio frequency amplifier stage.

One of the great advantages of the present arrangement is that it can be utilized in a receiver of moderate sensitivity which gives the best quality of reproduction for local signals, the throwing of a single switch making the receiver much more sensitive, and the quality of reproduction, though impaired to some extent, being still as good as that of any receiver which employs square-law detection.

While I have indicated and described one arrangement for carrying my invention into efiect,'itwillbe'apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and describedfbut that many modifications may be made without departing from the scope of my invention as set forth in the ap and means variably coupling the detector:

means and signal amplifier means, said con pling'including-Ineans to vary the characteristic of the detector means to follow'a linear law or a square law whenthe amplitude of signal energy impressed upon the detector input is varied-between predeterminedvalues.

2. A radio receiver comprising adetector stage including a space discharge triode, anaudio frequency amplifier of one or more stages, a variable resistance coupling between the detector and the first audio stage, said coupling including means to automatically vary the operating characteristic of the detector when the amplitude of signal energy impressed upon the detector input is varied between predetermined values.

3. A radio receiver consisting of means for collecting signal energy, a tuned radio amplifier, detecting means, audio amplifying means, signal utilization means, and selective means coupling the detecting means and audio amplifier for varying the'detection characteristic in accordance with the signal intensity level.

4. A radio receiver consisting of means for collecting signal energy, a tuned radio amplifier, detecting means, audio amplifying means, signal utilization means, and selective means including an adjustable resistor coupling the detecting means and audio amplifier for varying the detection characteristic in accordance with the signal intensity level.

5. In combination, a detector circuit, an audio frequency amplifier circuit, and a variable resistance couphng, between the detector output and the amplifier input, having means which changes the detection characteristic when the amplitude of signal energy impressed upon the input of the detector circuit is varied between predetermined values.

6. In a radio receiver, a radio frequency amplifier circuit, a detector stage including an electron discharge tube, and at least one sta e of audio frequency amplification incluc ing an electron discharge tube, an impedance coupling the detector tube output circuit and the audio amplifier tube input circuit, means for varying the magnitude of said impedance coupling between minimum and maximum values corresponding to high and low signal energy levels, the detector stage including means whereby the detector tube operates as a linear detector when said coupling is adjusted to its minimum value, and as a square law detector when said coupling is adjusted to its maximumvalue.

7 In a radio receiver, a detector having an output circuit, a succeeding amplifier provided with an input circuit, a resistor, a condenser, and local-distant control switching means for said output circuit connected with said resistor and said condenser and arranged to connect said resistor in shunt to said amplifier input circuit and said condenser in series between the resistor and the amplifier input circuit for distant reception.

. 8. In a radio receiver, a detector having an output circuit, an amplifier provided with an input circuit, a resistor connected in shunt to said output circuit, said resistor being provided with at least two taps, a condenser coupled to the amplifier input circuit, and a local-distant control switch movable in one position to connect the condenser to one of said taps and to another position to connect the other tap to said condenser.

' RENE A. BRADEN. 

